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484                          .           THE  JERBOA                                     CHAP.

Allied to this family, and indeed united with it by Tullberg,
but kept separate by Thomas, is the

Fam, 6. Heteromyidae.—The members of this family are also
American, but are not confined to the northern-central regions
of that continent, for the genus Heteromys extends into South
America,

The genus Dipodomys, with twelve species, is of a Jerboa-like
form, as the following measurements of an example of D. merriami
will show. The length of the head and body was 85 mm.; of
the tail 127 mm.; the hind-foot is 32 mm. It has but four
toes. The hind-limb is longer than the front-limbs.

In Perodipus the same form is exhibited. There are, however,
five toes, and the sole of the foot is hairy. The axis vertebra
and the two following vertebrae are fused together.

Perognathus is a third genus. It has the same general slender
form, but the tail is not so long, being but little longer than the
body. The hind-limbs, too, are shorter. The teats of this and
of Perodipus are as in Geomys. The two remaining genera of the
family are Heteromys and Microdipodops,

Fam» 7. Dipodidae.—This family consists of small, plain-
living, and leaping or arboreal creatures, commonly known as

FIG. 238.—Jerboa.    Dipus hirtvpes.     x J.    Eastern Europe.
Jerboas. The main anatomical characters of the family are the
following :-r-There is a large infra-orbital foramen. The molars
are always reduced, the premolar being either absent in the
lower jaw alone or in both jaws. This family presents an
obvious likeness to Dipodomys (hence the name of the latter) and
to some other members of the American familv" Heteromvidae